Method and apparatus for controlling and integrated fuel cell system

ABSTRACT

A control algorithm for operating an integrated fuel cell system includes the following steps: determining whether a power output of a fuel cell is within a first predetermined range of an electrical load coupled to the fuel cell; lowering a reactant flow to the fuel cell when the power output is within the first predetermined range; detecting an increase of the electrical load; determining whether the increase exceeds a second predetermined range; and increasing a reactant flow to the fuel cell when the increase exceeds the second predetermined range.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 USC 119(e) from U.S.Provisional Application No. 60/294,710, filed May 31, 2001, naming Jonesand Parks inventors, and titled “METHOD AND APPARATUS FOR CONTROLLING ANINTEGRATED FUEL CELL SYSTEM.” That application is incorporated herein byreference in its entirety and for all purposes.

BACKGROUND

[0002] The invention generally relates to an integrated fuel cell systemand associated methods for controlling the system.

[0003] A fuel cell is an electrochemical device that converts chemicalenergy produced by a reaction directly into electrical energy. Forexample, one type of fuel cell includes a polymer electrolyte membrane(PEM), often called a proton exchange membrane, that permits onlyprotons to pass between an anode and a cathode of the fuel cell. At theanode, diatomic hydrogen (a fuel) is reacted to produce protons thatpass through the PEM. The electrons produced by this reaction travelthrough circuitry that is external to the fuel cell to form anelectrical current. At the cathode, oxygen is reduced and reacts withthe protons to form water. The anodic and cathodic reactions aredescribed by the following equations:

[0004] H₂→2H⁺2e⁻ at the anode of the cell, and

[0005] O₂+4H⁺+4e⁻→2H₂O at the cathode of the cell.

[0006] A typical fuel cell has a terminal voltage of up to about onevolt DC. For purposes of producing much larger voltages, multiple fuelcells may be assembled together to form an arrangement called a fuelcell stack, an arrangement in which the fuel cells are electricallycoupled together in series to form a larger DC voltage (a voltage near100 volts DC, for example) and to provide more power.

[0007] The fuel cell stack may include flow field plates (graphitecomposite or metal plates, as examples) that are stacked one on top ofthe other. The plates may include various surface flow field channelsand orifices to, as examples, route the reactants and products throughthe fuel cell stack. A PEM is sandwiched between each anode and cathodeflow field plate. Electrically conductive gas diffusion layers (GDLs)may be located on each side of each PEM to act as a gas diffusion mediaand in some cases to provide a support for the fuel cell catalysts. Inthis manner, reactant gases from each side of the PEM may pass along theflow field channels and diffuse through the GDLs to reach the PEM. ThePEM and its adjacent pair of catalyst layers are often referred to as amembrane electrode assembly (MEA). An MEA sandwiched by adjacent GDLlayers is often referred to as a membrane electrode unit (MEU).

[0008] A fuel cell system may include a fuel processor that converts ahydrocarbon (natural gas or propane, as examples) into a fuel flow forthe fuel cell stack. For a given output power of the fuel cell stack,the fuel flow to the stack must satisfy the appropriate stoichiometricratios governed by the equations listed above. Thus, a controller of thefuel cell system may monitor the output power of the stack and based onthe monitored output power, estimate the fuel flow to satisfy theappropriate stoichiometric ratios. In this manner, the controllerregulates the fuel processor to produce this flow, and in response tothe controller detecting a change in the output power, the controllerestimates a new rate of fuel flow and controls the fuel processoraccordingly.

[0009] The fuel cell system may provide power to a load, such as a loadthat is formed from residential appliances and electrical devices thatmay be selectively turned on and off to vary the power that is demandedby the load. Thus, the load may not be constant, but rather the powerthat is consumed by the load may vary over time and abruptly change insteps. For example, if the fuel cell system provides power to a house,different appliances/electrical devices of the house may be turned onand off at different times to cause the load to vary in a stepwisefashion over time. Fuel cell systems adapted to accommodate variableloads are sometimes referred to as “load following” systems.

[0010] There is a continuing need for integrated fuel cell system andassociated methods for controlling such system designed to achieveobjectives including the forgoing in a robust, cost-effective manner.

SUMMARY

[0011] The invention provides control systems and algorithms for loadfollowing fuel cell systems. In one aspect, a control network isprovided for an integrated fuel cell system. A fuel cell has an outputpower that is characterized by a voltage and a current. A controller isadapted to vary a reactant flow to the fuel cell (e.g., by modulating avariable output blower or a valve on a pressure vessel). An electricalload (e.g., a power requirement of a collection of electrical appliancesdrawing power from the fuel cell) is connected to the fuel cell suchthat the fuel cell output power is supplied to the electrical load. Anelectrical load sensor is adapted to communicate a measurement of theelectrical load to the controller. The controller is adapted to monitora change in the electrical load (e.g., an increase or decrease in thecurrent demanded), and the controller is further adapted to vary thereactant flow when the change in the electrical load exceeds a firstpredetermined level. The controller is further adapted to delay varyingthe reactant flow for a first predetermined period.

[0012] The invention preferably applies to PEM fuel cells, but it canalso apply to other types of fuel cells, including solid oxide,phosphoric acid, etc. The reactant flow can be hydrogen or oxygen, orhydrogen rich fuel stream such as reformate, or oxygen rich streams suchas air.

[0013] The predetermined level which the load must exceed may be definedin the controller in a number of ways to ensure robust load followingperformance and to reduce signal noise. For example, the predeterminedlevel can be 110 percent of the fuel cell output power (or some otherpercentage), or alternatively, it can be a time period of 1 second (orsome other time period).

[0014] A fuel cell voltage sensor can be associated with the system thatis adapted to communicate a voltage of the fuel cell to the controller.The controller can then be configured to increase the reactant flow whenthe voltage of the fuel cell is below a second predetermined leveldefined in the controller. In some embodiments, the threshold valuesused by the controller to determine action can be programmed into thecontroller via software or user input. The controller can thus include acomputer readable memory, and the controller can be adapted to store areactant flow instruction referenced to a fuel cell electrical outputparameter. In other embodiments, such values used by the controller todetermine action can be input to the controller via firmware. In yetother embodiments, such values can be determined dynamically by thecontroller.

[0015] In some embodiments, the controller is adapted to lower thereactant flow until the voltage of the fuel cell is at least as low asthe second the predetermined level. For example, subject to othercriteria, such as a control logic requirement of maintaining a desiredfuel cell voltage or power output, the controller can step down areactant blower to avoid an inefficient situation of flowing too high adegree of excess reactants through the fuel cell for a given powerdemand.

[0016] The control network can further include a supplemental powersource (e.g., a battery or a utility power grid), where the controlleris adapted to supply power to the electrical load from the supplementalpower source when the electrical load exceeds a third predeterminedlevel. As an example, the controller can be adapted to supply power tothe electrical load from the supplemental power source during thepredetermined period.

[0017] In anther aspect, the invention provides a method of controllingan integrated fuel cell system, including the following steps:determining whether a power output of a fuel cell is within a firstpredetermined range of an electrical load coupled to the fuel cell;lowering a reactant flow to the fuel cell when the power output iswithin the first predetermined range; detecting an increase of theelectrical load; determining whether the increase exceeds a secondpredetermined range; and increasing a reactant flow to the fuel cellwhen the increase exceeds the second predetermined range.

[0018] Embodiments of such methods may further include other steps orfeatures as discussed herein, either alone or in combination. Forexample, a voltage of the fuel cell can be measured and communicated tothe controller, and the reactant flow can be increased when the voltageof the fuel cell is below a second predetermined level (e.g., 0.6volts). Methods may also include decreasing the reactant flow until thevoltage of the fuel cell is at least as low as the second thepredetermined level. As another feature, reactant flow instructionsreferenced to a fuel cell electrical output parameter can be stored in acomputer readable memory.

[0019] The electrical load can be supplied power from a supplementalpower source when the electrical load exceeds a third predeterminedlevel (e.g., of a magnitude that the fuel cell will not be able torespond to immediately, such as a transient load spike). Power can alsobe supplied to the electrical load from the supplemental power sourceduring a predetermined period when the electrical load exceeds the thirdpredetermined level. For example, the fuel cell system may take time toincrease reactant output (e.g., from a reformer) to accommodate a loadincrease, and the controller can be adapted to accommodate such a “waitperiod” through reliance on the supplemental power source. Anotherconsideration might be that a supplemental power source such as abattery may have a limited ability to respond to a power demand, so thecontroller can be configured to use the supplemental power sourceaccording to its performance properties.

[0020] In another aspect, a method is provided of controlling anintegrated fuel cell system, including the following steps: determiningwhether a power output of a fuel cell is within a first predeterminedrange of an electrical load coupled to the fuel cell; executing a steadystate algorithm when the power output is within the predetermined range;executing an up-transient algorithm when the power output is lower thanthe predetermined range; executing a down transient algorithm when thepower output is greater than the predetermined range; wherein the steadystate algorithm comprises maintaining a reactant flow above apredetermined level; wherein the up-transient algorithm comprisesincreasing the reactant flow; and wherein the down-transient algorithmcomprises decreasing the reactant flow.

[0021] Embodiments of such methods may further include other steps orfeatures as discussed herein, either alone or in combination. Forexample, a voltage of the fuel cell can be measured and communicated tothe controller; and the reactant flow can be increased when the voltageof the fuel cell is below a second predetermined level. The reactantflow can be decreased until the voltage of the fuel cell is at least aslow as the second the predetermined level. A reactant flow instructioncan be stored and referenced to a fuel cell electrical output parameterin a computer readable memory.

[0022] Such methods may also include supplying power to the electricalload from a supplemental power source when the electrical load exceeds athird predetermined level. Alternatively, methods may include supplyingpower to the electrical load from a supplemental power source during apredetermined period when the electrical load exceeds a thirdpredetermined level.

[0023] Advantages and other features of the invention will becomeapparent from the following description, drawing and claims.

DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic diagram of an integrated fuel cell system.

[0025]FIG. 2 is a schematic diagram of a control system for anintegrated fuel cell system.

[0026]FIG. 3 is a flow chart for a method of operating a fuel cellsystem under the present invention.

[0027]FIG. 4 is a flow chart for a method of operating a fuel cellsystem under the present invention.

DETAILED DESCRIPTION

[0028] The term “integrated fuel cell system” generally refers to a fuelcell stack that is coupled to components and subsystems that support theoperation of the stack. For example, this could refer to a fuel cellstack that is connected to a power conditioning device that convertsdirect current from the fuel cell into alternating current similar tothat available from the grid. It might also refer to a system equippedwith a fuel processor to convert a hydrocarbon (e.g., natural gas,propane, methanol, etc.) into a hydrogen rich stream (e.g., reformate)for use in the fuel cell. An integrated fuel cell system may alsoinclude a control mechanism to automate at least some portion of theoperation of the system. Integrated fuel cell systems may include asingle controller common to the entire system, or may include multiplecontrollers specific to various parts of the system. Likewise, theoperation of integrated fuel cell systems may be fully or partiallyautomated. Also, an integrated fuel cell system may or may not be housedin a common enclosure.

[0029] As one possible example, an integrated fuel cell system could bebased on four main systems: fuel processor, fuel cell stack, powerconditioner, and system controller. Other systems may be configured witha fuel source such that a fuel processor is not needed, as an example.In the following discussion the reference to these main systemcategories is intended for illustrative purposes only, since the controlalgorithms of the present invention are generally suitable to a largerange of fuel cell system configurations. The invention is not limitedby how the conceptual or physical boundaries of such systems aredefined.

[0030] Referring to FIG. 1, an integrated fuel cell system 100 is shown.Natural gas is injected into the system through conduit 102. The naturalgas flows through desulfurization vessel 104, which contains asulfur-adsorbent material such as activated carbon. The de-sufurizednatural gas is then flowed to a conversion reactor 110 via conduit 105.Before being reacted in the conversion reactor 110, the de-sulfurizednatural gas is mixed with air 106 and steam 108. It will be appreciatedthat the conversion reactor 110 is an auto thermal reactor. Theconverted natural gas, referred to as reformate, then flows through aseries of high temperature shift reactors 112 and 114, through a lowtemperature shift reactor 116, and then through a PROX reactor 118. Itwill be appreciated that the primary function of this series of reactorsis to maximize hydrogen production while minimizing carbon monoxidelevels in the reformate. The reformate is then flowed via conduit 120 tothe anode chambers (not shown) of a fuel cell stack 122.

[0031] Air enters the system via conduit 124 and through conduit 106 aspreviously mentioned. In the present example, the fuel cell stack 122uses sulfonated fluourocarbon polymer PEMs that need to be kept moistduring operation to avoid damage. While the reformate 120 tends to besaturated with water, the ambient air 124 tends to be subsaturated. Toprevent the ambient air 124 from drying out the fuel cells in stack 122,the air 124 is humidified by passing it through an enthalpy wheel 126,which also serves to preheat the air 124. The theory and operation ofenthalpy wheels are described in U.S. Pat. No. 6,013,385, which ishereby incorporated by reference. The air 124 passes through theenthalpy wheel 126 through the cathode chambers (not shown) of the fuelcell stack 122 via conduit 125. The air 124 picks up heat and moisturein the stack 122, and is exhausted via conduit 128 back through theenthalpy wheel 126. The enthalpy wheel 126 rotates with respect to theinjection points of these flows such that moisture and heat from thecathode exhaust 128 is continually passed to the cathode inlet air 124prior to that stream entering the fuel cell.

[0032] The anode exhaust from the fuel cell is flowed via conduit 130 toan oxidizer 132, sometimes referred to as an “anode tailgas oxidizer”.The cathode exhaust leaves the enthalpy wheel 126 via conduit 134 and isalso fed to the oxidizer 132 to provide oxygen to promote the oxidationof residual hydrogen and hydrocarbons in the anode exhaust 130. Asexamples, the oxidizer 132 can be a burner or a catalytic burner(similar to automotive catalytic converters). The exhaust of theoxidizer is vented to ambient via conduit 136. The heat generated in theoxidizer 132 is used to convert a water stream 138 into steam 108 thatis used in the system.

[0033] Referring to FIG. 2, a schematic is shown of a control system foran integrated high temperature PEM fuel cell system. Such a controlsystem can include the following components, as examples: (200) anelectronic controller, e.g., a programmable microprocessor; (202) agraphical user interface; (204) software for instructing the controller;(206) an air blower for providing the system with air, e.g., the fuelcell cathode and/or the fuel processor; (208) a fuel blower for drivinghydrocarbon into the fuel processor; (210) a stack voltage scanner formeasuring the stack voltage and/or the individual voltages of fuel cellswithin the stack; (212) a coolant pump for circulating a coolant throughthe fuel cell stack to maintain a desired stack operating temperature;(214) a coolant radiator and fan for expelling heat from the coolant toambient; (216) a fuel processor inlet air by-pass valve for controllingthe amount of air fed to the fuel processor; and (218) an oxidizer inletair control valve.

[0034] Such a control system can operate to control the followingvariables, as examples: (220) the fuel processor inlet oxygen to fuelratio; (222) the fuel processor inlet water to fuel ratio; (224) a fuelprocessor reactor temperature; (226) the voltage of the fuel cell stackor of individual fuel cells within the stack; (228) the oxidizertemperature; (230) electrical demand on the fuel cell system; (232) thecathode air stoich; (234) the anode fuel stoich; and (236) the systemcoolant temperature.

[0035] Exemplary fuel processor systems are described in U.S. Pat. Nos.6,207,122, 6,190,623, and 6,132,689, which are hereby incorporated byreference. For instance, in the case of a natural gas fuel processor,the system may include a variable speed blower for injecting natural gasinto the system, and a variable speed air blower for injecting air intothe system. The gas and air may be mixed in a mixing chamber, humidifiedto a desired level (e.g., the system may include some method of steamgeneration), and be preheated (e.g., in a gas/gas heat exchanger withheat from product gas from the fuel processor). The reactant mixture maythen be reacted in an auto thermal reactor (ATR) to convert the naturalgas to synthesis gas (H₂O+CH₄→3H₂+CO; ½O₂+CH₄→2H₂+CO). The fuelprocessor may also include a shift reactor (CO+H₂O→H₂+C0 ₂) to shift theequilibrium of the synthesis gas toward hydrogen production. The fuelprocessor may include multiple shift reactor stages.

[0036] Some fuel processor systems may also include a preferentialoxidation (PROX) stage (CO+½O₂→CO₂) to further reduce carbon monoxidelevels. The PROX reaction is generally conducted at lower temperaturesthat the shift reaction, such as 100-200° C. Like the CPO reaction, thePROX reaction can also be conducted in the presence of an oxidationcatalyst such as platinum. The PROX reaction can typically achieve COlevels less than 100 ppm. Other non-catalytic CO reduction and reformatepurification methods are also known, such as membrane filtration andpressure swing adsorption systems.

[0037] In some embodiments, the auto thermal reactor can be replaced bya reforming reactor (e.g., utilizing the endothermic steam reformingreaction: H₂O+CH₄→3H₂+CO), or by a catalytic partial oxidation reactor(CPO reactor: ½O₂+CH₄→2H₂+CO), which is exothermic. These terms aresometimes used loosely or interchangeably. In general, an auto thermalreactor is a reactor that combines the reforming and catalytic partialoxidation reactions to achieve a balance between the respectiveendothermic and exothermic elements. It should be noted that fuelprocessors are sometimes generically referred to as reformers, and thefuel processor output gas is sometimes generically referred to asreformate, without respect to the reaction that is actually employed.

[0038] The ATR catalyst can be a ceramic monolith that has beenwash-coated with a platinum catalyst (as known in the art, e.g.,operating at over 600° C.). The shift catalyst can also be platinumwash-coated ceramic monolith (e.g., operating between 300-600° C.). Theshift reactor can also include a catalyst that is operable at lowertemperatures. Other suitable catalyst and reactor systems are known inthe art.

[0039] In some embodiments, a desulfurization stage may be placedupstream from the fuel processor to remove sulfur compounds from thefuel before it is reacted (e.g., to avoid poisoning the catalysts of thefuel processor and/or the fuel cell stack). For example, activatedcarbon, zeolite, and activated nickel materials are all known in the artfor such application.

[0040] As known in the art, it may be desirable to control the water tofuel ratio (e.g., steam to carbon ratio) that is fed to the ATR. Forexample, it may be desirable to provide on average at least two watermolecules for every carbon atom provided in the fuel to prevent coking.It may also be desirable in some embodiments to adjust the air stoichthrough the fuel cell stack to control the amount of oxygen that isintroduced into the fuel processor with respect to the amount of fuelthat is introduced (e.g., O₂:CH₄ ratio, which can effect the operationtemperature of the ATR as an example).

[0041] Suitable fuel cell stack designs are well known. For example, thefuel cell systems taught in U.S. Pat. Nos. 5,858,569, 5,981,098,5,998,054, 6,001,502, 6,071,635, 6,174,616, and 09/502,886 are eachhereby incorporated by reference. In an integrated fuel cell system, thefuel cell stack may be associated with additional components andsubsystems. A coolant system may be used to circulate a liquid coolantthrough the stack to maintain a desired operating temperature. Aradiator or other heat transfer device may be placed in the coolant pathto provide coolant temperature control. The coolant may also performheat transfer in other areas of the system, such as in the fuelprocessor, or cooling reactants exiting the fuel processor to a desiredtemperature before entering the fuel cell stack. As an example, thecoolant may be circulated by a variable speed pump.

[0042] The reactant delivery system associated with the fuel cell stackmay include a variable speed air blower or compressor, and variableposition valves and/or orifices to control the amount and pressure offuel and air provided to the stack, as well as the ratio between thetwo. For a given electrical load, a certain amount of reactants must bereacted in the fuel cell to provide the power demanded by the load. Inthis sense, the amount of air and fuel supplied to the fuel cell stackmay each be referred to in terms of stoichiometry (i.e., thestoichiometric equations associated with the fuel cell reactions:H₂→2H⁺+2e³¹; and O₂+4H⁺+4e⁻→2H₂O). For example, supplying 1 “stoich” ofreformate means that enough reformate is supplied to the fuel cell stackto satisfy the power demand of the load, assuming that all of thehydrogen in the reformate reacts. However, since not all of the hydrogenin the reformate will actually react, the fuel may be supplied at anelevated stoich (e.g., 2 stoich would refer to twice this amount) toensure that the amount that actually will react will be enough to meetthe power demand. Similarly, air may also be supplied to the fuel cellstack in excess of what is theoretically needed (e.g., 2 stoich).

[0043] The reactant plumbing associated with the stack may be conductedin part by a manifold. For example, the teachings of U.S. Pat. No.09/703,249 are hereby incorporated by reference. Such a manifold may befurther associated with a water collection tank that receives condensatefrom water traps in the system plumbing. The water tank may include alevel sensor. Some fuel cell systems may require an external source ofwater during operation, and may thus include a connection to a municipalwater source. A filter may be associated with the connection from themunicipal water supply, such as a particulate filter, a reverse osmosismembrane, a deionization bed, etc.

[0044] Some fuel cell membranes, such as those made from sulfonatedflourocarbon polymers, require humidification. For example, it may benecessary to humidify reactant air before it is sent through the fuelcell in order to prevent drying of the fuel cell membranes. In suchsystems, a reactant humidification system may be required. It will beappreciated that in systems utilizing reformate, this generally refersto humidifying only the air fed to the fuel cell stack and not the fuelstream, since the reformate exiting the fuel processor is generallysaturated. One method of humidification is to generate steam which issupplied to a reactant stream. Membrane humidification systems are alsoknown, as well as enthalpy wheel systems, as taught in U.S. Pat. No.6,013,385, which is hereby incorporated by reference.

[0045] The spent fuel exhausted from the fuel cell stack may containsome amount of unreacted hydrogen or unreacted hydrocarbon or carbonmonoxide from the fuel processor. Before the spent fuel is vented to theatmosphere, it may be sent through an oxidizer to reduce or remove suchcomponents. Suitable oxidizer designs are known, such as burner designs,and catalytic oxidizers similar to automotive catalytic converters.Oxidizers may utilize air exhausted from the fuel cell stack, and mayhave an independent air source, such as from a blower. In some systems,the heat generated by the oxidizer may be used, for example, to generatesteam for use in the fuel processor or to humidify the fuel cellreactants.

[0046] Another system that may be associated with the fuel cell stack isa mechanism for measuring the voltages of the individual fuel cellswithin the stack. For example, the teachings of U.S. Pat. Nos.6,140,820, 09/379,088, 09/629,548, 09/629,003 are each herebyincorporated by reference. In some systems, the health of a fuel cellstack may be determined by monitoring the individual differentialterminal voltages (also referred to as cell voltages) of the fuel cells.Particular cell voltages may individually vary under load conditions andcell health over a range from −1 volt to +1 volt, as an example. Thefuel cell stack typically may include a large number of fuel cells(between 50-100, for example), so that the terminal voltage across theentire stack is the sum of the individual fuel cell voltages at a givenoperating point. As the electrical load on the stack is increased, some“weak” cells may drop in voltage more quickly than others. Driving anyparticular cell to a low enough voltage under an electrical load candamage the cell, so systems may include a mechanism for coordinating thecell voltages with the electrical demand and reactant supply to the fuelcell stack. For example, the teachings of U.S. Pat. Nos. 09/749,261,09/749,297 are hereby incorporated by reference.

[0047] A fuel cell stack typically produces direct current at a voltagewhich varies according to the number of cells in the stack and theoperating conditions of the cells. Applications for the power generatedby a fuel cell stack may demand constant voltage, or alternating currentat a constant voltage and frequency similar to a municipal power grid,etc. Integrated fuel cell systems may therefore include a powerconditioning system to accommodate such demands. Technologies forconverting variable direct current voltages to constant or relativelyconstant voltages are well known, as are technologies for invertingdirect currents to alternating currents. Suitable power conditionertopologies for fuel cells are also well known. For example, theteachings of U.S. Pat. No. 09/749,297 are hereby incorporated byreference.

[0048] A battery system may also be associated with the powerconditioning system, for example, to protect the fuel cells from fuelstarvation upon sudden electrical load increases on the stack. A batterysystem can also be used, as examples, to supplement the peak outputpower of the fuel cell system, or to provide continuous power to anapplication while the fuel cell system is temporarily shut down (as forservicing) or removed from the load. The battery system may also includea system for periodically charging the batteries when necessary.

[0049] Some fuel cell systems may be operated independently from thepower grid (grid independent systems), while other fuel cell systems maybe operated in conjunction with the power grid (grid parallel systems).For grid parallel systems, the system may include a transfer switch totransfer the electrical load between the fuel cell system and the powergrid. For example, in some grid parallel systems, the electrical loadcan be switched from the fuel cell system to the grid when the fuel cellsystem needs to be shut down for maintenance. In still other gridparallel systems, the electrical load can be shared between the fuelcell system and the grid. The fuel cell can also be used to feed powerto the grid (in this sense, the grid may be referred to as a “sink”),while an appliance takes its power from the grid. Other arrangements arepossible.

[0050] System controllers may automate the operation of fuel cellsystems to varying degrees, and may have varying capacities foradjustment and reconfiguration. For example, some controllers may relyin part on software for instruction sets to provide enhanced flexibilityand adaptability, while other controllers may rely on hardware toprovide enhanced reliability and lower cost. Control systems may alsoinclude combinations of such systems. Controllers may include analgorithm that coordinates open and closed loop functions. In thiscontext, an open loop function is one that does not utilize feedback,such as adjusting a blower according to a look-up table withoutverifying the effect of the adjustment or iterating the adjustmenttoward a desired effect. A closed loop function is one that utilizesfeedback to iterate adjustments toward a desired effect.

[0051] In general, the controller circuitry may include data inputs fromsystem components such as safety sensors and thermocouples throughoutthe system. As an example, such data inputs may report data in the formof variable voltage or current signals, or as binary on/off signals. Thecontroller circuitry may also include devices to control the voltageand/or current supplied to various components in the system, for exampleto control variable speed pumps and blowers. The power supplied tosystem components may be referred to as the parasitic load.

[0052] In one embodiment, a control system to enable an integrated fuelcell system to follow a variable electrical load is based on fouroperating states. In a first state (steady state), there are either noload changes, or the load changes that occur are within a “dead band”threshold range in which no action is taken. Essentially, in this statethe system operates at steady state. In a second state (up transient),in response to an increased load, a control signal is sent to the fuelprocessor to increase the amount of reformate provided to the fuel cellstack. In a third state (down transient), a control signal is sent tothe fuel processor to decrease the amount of reformate provided to thefuel cell stack. In a fourth state (equilibration), the controllerdelays changes in operating state for a certain amount of time.

[0053] In another embodiment, an additional stoich optimization statemay be coordinated wherein one or both reactant stoichs are optimized bylowering them as far as possible without lowering the stack voltage or aweak cell voltage below a desired point. See, e.g., U.S. Pat. No.09/749,298, which is hereby incorporated by reference. It will beappreciated that the performance of the fuel cell stack can vary and/ordegrade over time, and can vary from stack to stack, so it may bedesirable to treat the amount of reformate needed for a given electricalload as a dynamic variable. In some embodiments, the control algorithmcan include a learning function where the stoich optimization functiondescribed above is used to “teach” the system what reactant stoichs areappropriate for a given electrical demand or for a given system. As anexample, such a learning function can populate a dynamic look-up tablethat is used by the system as a starting point for requesting an amountof reformate to satisfy an electrical load. As another example, thelearning function can include a correction factor for the stack powerdemand and/or reformate demand signals to adjust hydrogen and/or oxygenstoichs according to the optimized values. Also, in this way, the systemcan minimize the frequency that the stoich optimization step isnecessary.

[0054] Steady State

[0055] In some embodiments, the steady state mode may include a logicaloperation that determines whether the stack is producing enough power tosatisfy the electrical load on the stack. For example, the size of theelectrical load on the stack could be measured (e.g., in terms of poweror current demand at a given voltage, etc.) and a controller couldcompare this value to a measured power output of the stack (otherparameters can also be measured: stack voltage, cell voltages, stackcurrent, etc.). If the stack is not producing enough power, thecontroller is shifted to the up-transient state. If the stack isproducing more than enough power, the controller is shifted to thedown-transient state. In some embodiments, the difference between theamount demanded by the load and the amount supplied by the stack can becompared to a “dead-band” range in which no action is taken.

[0056] Up-Transient State

[0057] In general, the up transient state includes a logical operationthat increases the output of the fuel processor (e.g., by increasing theflows of reactants to the fuel processor) until the power supplied bythe stack is enough to satisfy the electrical load on the stack.

[0058] In some embodiments, the reformer may only be able to respond toup transients of a certain size without affecting the outlet gascomposition. For example, if an up-transient signal causes a suddenincrease in fuel beyond a certain range, the reformer may produceelevated amounts of carbon monoxide or other undesired components untilthe reformer reaches equilibrium at the new set point. In suchembodiments, the up-transient state may include a logical operation tocompare the difference between stack output and electrical demand with avalue representing a maximum acceptable up-transient step size. In someembodiments, the up-transient state may include a logical operation tocompare the difference between stack output and electrical demand with avalue representing a maximum acceptable up-transient change rate. Thus,by controlling the up-transient rate of the reformer, the abovesituation (e.g., temporary elevated CO level) can be avoided.

[0059] In some embodiments, in the up-transient state, the controllercan coordinate the supply of power between the fuel cell stack and abattery system. For example, battery systems may be used to supplementpower supplied by the fuel cell stack if the fuel cell stack is unableto meet the transient (e.g., load increase is too high or too sudden).In some systems, the controller can place a limit on the amount of loadon the stack (e.g., a dynamic current limit), to prevent excessive loadfrom driving the cell voltages below acceptable levels due to fuelstarvation.

[0060] In some embodiments, the up-transient state can include a logicaloperation that accounts for reformer response time. For example, thecontroller in the up-transient state may use this response time to delaysending signals to a power conditioning system to determine factors suchas the amount of power that is supplied to the load and the amount ofpower that is taken from the stack. In addition, as previouslyindicated, the system may also employ a fourth equilibration state wherethe system delays changing operating states for a certain period oftime. In some cases, this period can correspond to the time it takes forthe system to equilibrate to the new operating point.

[0061] In some embodiments, the up-transient state may include a logicaloperation that increases the hydrogen stoich that the system requests inresponse to a given load. As an example, running at a higher than normalhydrogen stoich during a transient can increase voltage stability duringthe transient.

[0062] Down Transient

[0063] In general, the down-transient state includes a logical operationthat decreases the output of the fuel processor (e.g., by decreasing theflows of reactants to the fuel processor) until the power supplied bythe stack is appropriate to satisfy the electrical load on the stack.

[0064] In some embodiments, the reformer may only be able to respond todown transients of a certain size without affecting the outlet gascomposition. For example, if a down-transient signal causes a suddendecrease in fuel beyond a certain range, the reformer may produceelevated amounts of carbon monoxide or other undesired components untilthe reformer reaches equilibrium at the new set point. In suchembodiments, the down-transient state may include a logical operation tocompare the difference between stack output and electrical demand with avalue representing a maximum acceptable down-transient step size. Thus,by controlling the down-transient rate of the reformer, the abovesituation (e.g., temporary elevated CO level) can be avoided.

[0065] In some embodiments, the down-transient state can also include alogical operation that accounts for reformer response time. For example,the controller in the down-transient state may use this response time todelay decreasing the amount of reactants that are provided to thereformer, while sending signals to a power conditioning system withoutdelay (e.g., to determine factors such as the amount of power that issupplied to the load and the amount of power that is taken from thestack). In addition, as previously indicated, the system may also employa fourth equilibration state where the system delays changing operatingstates for a certain period of time. In some cases, this period cancorrespond to the time it takes for the system to equilibrate to the newoperating point. In some cases, the system can sink the excess power toa battery system or to another application such as sinking the power tothe grid.

[0066] In some embodiments, the down-transient state may include alogical operation that increases the hydrogen stoich that the systemrequests in response to a given load. As an example, running at a higherthan normal hydrogen stoich during a transient can increase voltagestability during the transient.

[0067] Control Algorithms

[0068] In another aspect, the invention provides a control apparatus forexecuting any of the above logic schemes for a load following fuel cellsystem, alone or in combination. Techniques for preparing circuitry toprovide electronic control systems are well known in the art, such thata system under the present invention with the features and aspectsdescribed above could be implemented by one of ordinary skill, forexample by reference in part to the patents mentioned above.

[0069] In another aspect of the invention, a method is provided forenabling a fuel cell system to accommodate variable demands for poweroutput. Such method may include any of the above logic schemes for aload following fuel cell system, alone or in combination.

[0070] In another aspect of the invention, an integrated load followingfuel cell system is provided with any of the features described above,either alone or in combination.

[0071] In another embodiment of the invention, an article of manufactureis provided that includes at least one computer usable medium havingcomputer readable code embodied thereon for enabling a fuel cell systemto accommodate variable demands for power output according to any of theabove logic schemes, alone or in combination.

[0072] In another embodiment, the invention provides at least oneprogram storage device readable by a machine, tangibly embodying atleast one program of instructions executable by the machine to perform amethod for enabling a fuel cell system to accommodate variable demandsfor power output according to any of the above logic schemes, alone orin combination.

[0073] In another aspect, the invention also provides methods foroperating fuel cell systems according to the concepts and featuresdiscussed herein. For example, referring to FIG. 3, such a method 300includes the following steps: (302) determining whether a power outputof a fuel cell is within a first predetermined range of an electricalload coupled to the fuel cell; (304) lowering a reactant flow to thefuel cell when the power output is within the first predetermined range;(306) detecting an increase of the electrical load; (308) determiningwhether the increase exceeds a second predetermined range; and (310)increasing a reactant flow to the fuel cell when the increase exceedsthe second predetermined range.

[0074] Additional embodiments of such methods may further includevarious other steps or features. For example, a voltage of the fuel cellcan be measured and communicated to the controller, and the reactantflow can be increased when the voltage of the fuel cell is below asecond predetermined level (e.g., 0.6 volts). Methods may also includedecreasing the reactant flow until the voltage of the fuel cell is atleast as low as the second the predetermined level. As another feature,reactant flow instructions referenced to a fuel cell electrical outputparameter can be stored in a computer readable memory.

[0075] The electrical load can be supplied power from a supplementalpower source when the electrical load exceeds a third predeterminedlevel (e.g., of a magnitude that the fuel cell will not be able torespond to immediately, such as a transient load spike). Power can alsobe supplied to the electrical load from the supplemental power sourceduring a predetermined period when the electrical load exceeds the thirdpredetermined level. For example, the fuel cell system may take time toincrease reactant output (e.g., from a reformer) to accommodate a loadincrease, and the controller can be adapted to accommodate such a “waitperiod” through reliance on the supplemental power source. Anotherconsideration might be that a supplemental power source such as abattery may have a limited ability to respond to a power demand, so thecontroller can be configured to use the supplemental power sourceaccording to its performance properties.

[0076] Referring to FIG. 4, another method 400 is shown for operating afuel cell system under the present invention, including the followingsteps: (402) determining whether a power output of a fuel cell is withina first predetermined range of an electrical load coupled to the fuelcell; (404) executing a steady state algorithm when the power output iswithin the predetermined range; (408) executing an up-transientalgorithm when the power output is lower than the predetermined range;(410) executing a down transient algorithm when the power output isgreater than the predetermined range; (406) wherein the steady statealgorithm comprises maintaining a reactant flow above a predeterminedlevel; (410) wherein the up-transient algorithm comprises increasing thereactant flow; and (414) wherein the down-transient algorithm comprisesdecreasing the reactant flow.

[0077] Embodiments of such methods may further include other steps orfeatures. For example, a voltage of the fuel cell can be measured andcommunicated to the controller, and the reactant flow can be increasedwhen the voltage of the fuel cell is below a second predetermined level.The reactant flow can be decreased until the voltage of the fuel cell isat least as low as the second the predetermined level. A reactant flowinstruction can be stored and referenced to a fuel cell electricaloutput parameter in a computer readable memory.

[0078] Such methods may also include supplying power to the electricalload from a supplemental power source when the electrical load exceeds athird predetermined level. Alternatively, methods may include supplyingpower to the electrical load from a supplemental power source during apredetermined period when the electrical load exceeds a thirdpredetermined level.

[0079] While the invention has been disclosed with respect to a limitednumber of embodiments, those skilled in the art, having the benefit ofthis disclosure, will appreciate numerous modifications and variationstherefrom. It is intended that the invention covers all suchmodifications and variations as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A control network for an integrated fuel cellsystem, comprising: a fuel cell having an output power, the fuel cellhaving a voltage and an output current; a controller adapted to vary areactant flow to the fuel cell; an electrical load connected to the fuelcell such that the fuel cell output power is supplied to the electricalload; an electrical load sensor adapted to communicate a measurement ofthe electrical load to the controller; wherein the controller is adaptedto monitor a change in the electrical load, and wherein the controlleris further adapted to vary the reactant flow when the change in theelectrical load exceeds a first predetermined level; and wherein thecontroller is further adapted to delay varying the reactant flow for afirst predetermined period.
 2. The control network of claim 1, whereinthe fuel cell is a PEM fuel cell.
 3. The control network of claim 1,wherein the reactant flow comprises hydrogen.
 4. The control network ofclaim 1, wherein the electrical load comprises a residential appliance.5. The control network of claim 1, wherein the predetermined level is110 percent of the fuel cell output power.
 6. The control network ofclaim 1, wherein the first predetermined period is less than 1 second.7. The control network of claim 1, further comprising a fuel cellvoltage sensor adapted to communicate a voltage of the fuel cell to thecontroller; and wherein the controller is adapted to increase thereactant flow when the voltage of the fuel cell is below a secondpredetermined level.
 8. The control network of claim 7, wherein thecontroller is further adapted to lower the reactant flow until thevoltage of the fuel cell is at least as low as the second thepredetermined level.
 9. The control network of claim 1, wherein thecontroller comprises a computer readable memory, and the controller isadapted to store a reactant flow instruction referenced to a fuel cellelectrical output parameter.
 10. The control network of claim 1, furthercomprising a supplemental power source, wherein the controller isadapted to supply power to the electrical load from the supplementalpower source when the electrical load exceeds a third predeterminedlevel.
 11. The control network of claim 1, further comprising asupplemental power source, wherein the controller is adapted to supplypower to the electrical load from the supplemental power source duringthe predetermined period.
 12. A method of controlling an integrated fuelcell system, comprising: determining whether a power output of a fuelcell is within a first predetermined range of an electrical load coupledto the fuel cell; lowering a reactant flow to the fuel cell when thepower output is within the first predetermined range; detecting anincrease of the electrical load; determining whether the increaseexceeds a second predetermined range; and increasing a reactant flow tothe fuel cell when the increase exceeds the second predetermined range.13. The method of claim 12, further comprising: measuring a voltage ofthe fuel cell and communicating the voltage to the controller;increasing the reactant flow when the voltage of the fuel cell is belowa second predetermined level.
 14. The method of claim 13, furthercomprising: decreasing the reactant flow until the voltage of the fuelcell is at least as low as the second the predetermined level.
 15. Themethod of claim 12, further comprising: storing a reactant flowinstruction referenced to a fuel cell electrical output parameter in acomputer readable memory.
 16. The method of claim 12, furthercomprising: supplying power to the electrical load from a supplementalpower source when the electrical load exceeds a third predeterminedlevel.
 17. The method of claim 12, further comprising: supplying powerto the electrical load from a supplemental power source during apredetermined period when the electrical load exceeds a thirdpredetermined level.
 18. A method of controlling an integrated fuel cellsystem, comprising: determining whether a power output of a fuel cell iswithin a first predetermined range of an electrical load coupled to thefuel cell; executing a steady state algorithm when the power output iswithin the predetermined range; executing an up-transient algorithm whenthe power output is lower than the predetermined range; executing a downtransient algorithm when the power output is greater than thepredetermined range; wherein the steady state algorithm comprisesmaintaining a reactant flow above a predetermined level; wherein theup-transient algorithm comprises increasing the reactant flow; andwherein the down-transient algorithm comprises decreasing the reactantflow.
 19. The method of claim 18, further comprising: measuring avoltage of the fuel cell and communicating the voltage to thecontroller; increasing the reactant flow when the voltage of the fuelcell is below a second predetermined level.
 20. The method of claim 19,further comprising: decreasing the reactant flow until the voltage ofthe fuel cell is at least as low as the second the predetermined level.21. The method of claim 18, further comprising: storing a reactant flowinstruction referenced to a fuel cell electrical output parameter in acomputer readable memory.
 22. The method of claim 18, furthercomprising: supplying power to the electrical load from a supplementalpower source when the electrical load exceeds a third predeterminedlevel.
 23. The method of claim 18, further comprising: supplying powerto the electrical load from a supplemental power source during apredetermined period when the electrical load exceeds a thirdpredetermined level.